TUBIFEX. 311 



group of ectodermal cells whose boundaries are indistinct, 

 but whose nuclei are large, round and nucleolated. 

 According to Vejdovsky, any of these cells are capable of 

 giving rise to new setae to replace those which may have 

 been lost. The free ends of the setae are seen constantly 

 to change their position as the worm rapidly coils and 

 uncoils itself. Sometimes they are directed forwards, then 

 backwards, and often, especially when the worm is quiet 

 for a moment, they are placed almost at right angles to 

 the body. When the worm is crawling forwards the setae 

 are always pointing somewhat backwards, but a sudderj 

 twist of the body is sufficient to change their position 

 completely. 



There are two sets of muscles by which the setae 

 are moved : — 



(a) The parieto-vaginal muscles, which are attached 

 to the base of the setigerous follicle and to the body wall 

 on all sides of it. These muscles regulate the movements 

 of the whole follicle. If those lying on the posterior side 

 of the sac contract, the inner ends of the setae are drawn 

 back, and the free ends are pushed forward, whereas, if 

 those muscles on the anterior side contract, the setae are 

 directed backwards (PI. II, fig. 5, m.p.). 



(b) The intrafollicular muscles (fig. 5, m.f.). These 

 are attached to the body wall and to the individual setae, 

 and thus the movements of the setae can be regulated. 



The development of the setae in this worm resembles, 

 in all essentials, that of the setae of other Oligochaeta, 

 and therefore it is only necessary to refer to it briefly. 

 The setae appear as small cones of chitinous substance at 

 the bottom of the sac. The apex of the seta is first developed 

 and then growth in length proceeds from the inner end 

 until the seta has pierced the body wall and attained its 

 full length. 



